Abstract:

Provided is a semiconductor laser, wherein (λa-λw)>15 (nm)
and Lt<25 (μm), where λw is the wavelength of light
corresponding to the band gap of the active layer disposed at a position
within a distance of 2 μm from one end surface in a resonator
direction, λa is the wavelength of light corresponding to the band
gap of the active layer disposed at a position that is spaced a distance
of equal to or more than ( 3/10)L and <( 7/10)L from the one end
surface in a resonator direction, "L" is the resonator length, and "Lt"
is the length of a transition region provided between the position of the
active layer with a band gap corresponding to a light wavelength of
λw+2 (nm) and the position of the active layer with a band gap
corresponding to a light wavelength of λa-2 (nm) in the resonator
direction.

Claims:

1. A method of manufacturing a semiconductor laser, comprising:forming an
active layer over a substrate; anddiffusing impurities into a region near
an end surface of said active layer to alloy said active layer,wherein
said alloying said active layer includes:preparing a semiconductor layer
A and a semiconductor layer B;sequentially forming said semiconductor
layer B and said semiconductor layer A over said active layer when the
solid solubility limit concentration of said semiconductor layer A with
respect to said impurities is Ma and the solid solubility limit
concentration of said semiconductor layer B with respect to said
impurities is Mb (Ma>Mb);forming a groove in said semiconductor layer
A between a region where said region near said end surface is to be
formed and a region where a region other than said region near said end
surface is to be formed; andforming a layer including said impurities so
as to come into contact with only the surface of said first semiconductor
layer A in said region where said region near said end surface is to be
formed or exposing only the surface of said semiconductor layer A in said
region where said region near said end surface is to be formed to a gas
including said impurities, and diffusing said impurities into said active
layer in said region where said region near said end surface is to be
formed through said semiconductor layer A and said semiconductor layer B.

2. The method of manufacturing a semiconductor laser as set forth in claim
1,wherein, when said semiconductor layer A is a cap layer and said
semiconductor layer B is a clad layer, said cap layer remains over said
clad layer in said region where said region other than said region near
said end surface is to be formed.

3. The method of manufacturing a semiconductor laser as set forth in claim
1,wherein, when said semiconductor layer A is a cap layer and said
semiconductor layer B is a clad layer, said cap layer remains over said
clad layer in said region where said region near said end surface is to
be formed.

4. The method of manufacturing a semiconductor laser as set forth in claim
1,wherein, when said semiconductor layer A is a cap layer and said
semiconductor layer B is a clad layer, said cap layer over said clad
layer is removed in said region where said region near said end surface
is to be formed, anda current block layer is formed over said clad layer.

5. The method of manufacturing a semiconductor laser as set forth in claim
4, wherein said current block layer is a single layer or a multi-layer
structure.

6. The method of manufacturing a semiconductor laser as set forth in claim
1,wherein said step of forming said groove includes:forming a layer that
prevents the diffusion of said impurities, in said groove.

7. The method of manufacturing a semiconductor laser as set forth in claim
1, wherein said impurities are Zn.

8. The method of manufacturing a semiconductor laser as set forth in claim
1, wherein a material forming said semiconductor layer B includes GaInP,
AlGaInP, or AlGaAs.

9. The method of manufacturing a semiconductor laser as set forth in claim
1, wherein a material forming said semiconductor layer A includes GaAs.

10. The method of manufacturing a semiconductor laser as set forth in
claim 1, wherein said layer including said impurities is a ZnO film
formed by a sputtering method.

11. The method of manufacturing a semiconductor laser as set forth in
claim 1, wherein said semiconductor laser is self-oscillated.

12. A method of diffusing impurities into a semiconductor layer B through
a semiconductor layer A, comprising:preparing said semiconductor layer A
and said semiconductor layer B;forming said semiconductor layer A over
said semiconductor layer B when the solid solubility limit concentration
of said semiconductor layer A with respect to said impurities is Ma and
the solid solubility limit concentration of said semiconductor layer B
with respect to said impurities is Mb (Ma>Mb);forming a groove in said
semiconductor layer A, to form a first semiconductor layer A and a second
semiconductor layer A over said semiconductor layer B; andforming a layer
including said impurities so as to come into contact with only the
surface of said first semiconductor layer A and diffusing said impurities
into said semiconductor layer B through said first semiconductor layer A.

13. The method of diffusing impurities as set forth in claim 12, wherein
said impurities are Zn.

14. The method of diffusing impurities as set forth in claim 12, wherein a
material forming said semiconductor layer B includes GaInP, AlGaInP, or
AlGaAs.

15. The method of diffusing impurities as set forth in claim 12, wherein a
material forming said semiconductor layer A includes GaAs.

16. A semiconductor laser comprising:a substrate; andan active layer that
is provided over said substrate, said active layer in a region near at
least one end surface is alloyed by the diffusion of impurities, the band
gap of said active layer in said region near said end surface is wider
than that of said active layer in a region other than said region near
said end surface, andwherein (λa-.lamda.w)>15 (nm) and Lt is
less than 25 (μm), where "λw (nm)" is the wavelength of light
corresponding to the band gap of said active layer disposed at a position
within a distance of 2 μm from one end surface in a resonator
direction, "λa" (nm) is the wavelength of light corresponding to
the band gap of said active layer disposed at a position that is spaced a
distance of equal to or more than ( 3/10)L and equal to or less than (
7/10)L from said one end surface in a resonator direction, "L" is said
resonator length, and "Lt" is the length of a transition region provided
between the position of said active layer with a band gap corresponding
to a light wavelength of λw+2 (nm) and the position of said active
layer with a band gap corresponding to a light wavelength of λa-2
(nm) in said resonator direction.

17. The semiconductor laser as set forth in claim 16, further comprising:a
clad layer that is provided over said active layer; anda cap layer that
is provided over said clad layer,wherein the solid solubility limit
concentration of said cap layer with respect to said impurities is higher
than that of said clad layer.

18. The semiconductor laser as set forth in claim 17, wherein a material
forming said cap layer includes GaAs.

19. The semiconductor laser as set forth in claim 17, wherein a material
forming said clad layer includes GaInP, AlGaInP or AlGaAs.

20. The semiconductor laser as set forth in claim 16, wherein a material
forming said active layer includes GaInP, AlGaInP or AlGaAs.

21. The semiconductor laser as set forth in claim 16, wherein said
impurities are Zn.

22. The semiconductor laser as set forth in claim 16, further comprising:a
current block layer that is provided over said active layer,wherein said
semiconductor laser is self-oscillated.

23. The semiconductor laser as set forth in claim 16, wherein the Lt of
said transition region is equal to or less than 12 μm and equal to or
more than 1 μm.

24. The semiconductor laser as set forth in claim 16, further comprising:a
ridge-shaped strip or a ridge-buried-shaped strip.

25. The semiconductor laser as set forth in claim 17, wherein a groove is
provided in said cap layer between said region near said end surface and
said region other than said region near said end surface.

26. The semiconductor laser as set forth in claim 16, wherein said
resonator length L is equal to or less than 500 μm.

Description:

[0001]The application is based on Japanese patent application No.
2009-046074, the content of which is incorporated hereinto by reference.

BACKGROUND

[0002]1. Technical Field

[0003]The present invention relates to a semiconductor laser and a method
of manufacturing a semiconductor laser.

[0004]2. Related Art

[0005]As a technique for preventing the deterioration of an end surface
due to optical damage in semiconductor lasers, a technique has been
proposed in which a window structure that does not absorb laser
oscillation light is provided at an end surface.

[0007]FIGS. 16A to 16C show a process of manufacturing the semiconductor
laser. First, a ZnO layer 201 is selectively etched and a p-GaAs contact
layer 109 where a gain region 004 is to be formed is selectively etched
(FIG. 16A). Then, a dielectric film 202 is formed on the entire surface
of a wafer (FIG. 16B). Then, a heat treatment (annealing) is performed to
diffuse Zn in the ZnO layer 201 into an active layer 104 in a solid phase
(FIG. 16c).

[0008]In this way, in the semiconductor laser, Zn is diffused from only a
portion coming into contact with the p-GaAs contact layer 109. Therefore,
the diffusion of impurities is prevented as compared to a structure in
which the impurities are diffused with a contact layer provided on the
entire surface.

[0009]In addition, Japanese Unexamined Patent Publication No. 2006-319120
also discloses a structure in which partition regions including
impurities are provided on both sides of a Zn-diffused region, thereby
preventing the diffusion of impurities into an active layer in gain
regions provided on both sides of a window region due to annealing.

[0010]In the semiconductor laser disclosed in Japanese Unexamined Patent
Publication No. 2006-319120, as shown in FIGS. 16A, 16B, and 16C, a
p-GaAs contact layer 109 is formed only in a region where a window region
is to be formed in which the ZnO layer 201 on the active layer 104 is
formed, and the p-GaAs contact layer 109 is removed from most of the gain
region. As described in the specification or the drawings of Japanese
Unexamined Patent Publication No. 2006-319120, in portions other than the
window, the subsequent manufacturing process is performed with the
surface of the p-type GaInP clad layer 107 being exposed.

[0011]However, it is known in this technical field that a defect is more
likely to occur in the vicinity of the surface of a layer made of GaInP
or AlGaInP than a layer made of GaAs due to, for example, a heat
treatment or a plasma process. Therefore, a crystal defect is likely to
occur in the exposed p-type GaInP clad layer 107 due to a thermal history
of a manufacturing process. The defect is spread into a crystal by the
heat treatment, and interdiffuses elements forming the crystal.
Therefore, in the active layer 104 in the gain region, alloying occurs
due to the interdiffusion of the crystal, which may result in a variation
in band gap.

[0012]Even though the diffusion of Zn in the lateral direction is
prevented, it is difficult to prevent the alloying of the active layer in
the gain region where the p-GaAs contact layer 109 is not provided with
high reproducibility.

[0013]Although the band gap of the active layer is not described in detail
in Japanese Unexamined Patent Publication No. 2006-319120, the band gap
of the active layer is likely to be widened due to alloying in the gain
region where the ZnO layer 201 is not formed.

[0014]Japanese Unexamined Patent Publication No. 2007-318077 discloses an
improved semiconductor laser that prevents the generation of a crystal
defect due to process damage.

[0015]FIGS. 17A to 17D show a process of manufacturing the semiconductor
laser. First, an n-type AlGaInP clad layer 3, an active layer 4, a first
p-type AlGaInP clad layer 5, a p-type etching stop layer 6, a second
p-type AlGaInP clad layer 7, a p-type barrier reduction layer 8, and a
p-type GaAs cap layer 9 are sequentially formed on an n-type GaAs
substrate 2, and the p-type GaAs cap layer 9 near the end surface of a
resonator is removed to form an opening. Then, a ZnO layer 11 is formed
in the opening, and a heat treatment is performed to diffuse Zn included
in the ZnO layer 11 into the active layer 4, thereby forming a window
region M. Then, a strip-shaped insulating film mask pattern 16 is formed
in a resonator direction so as to cover the window region M. Then, only
the p-type GaAs cap layer 9 is removed by a selective etching solution to
form a ridge portion 17. According to Japanese Unexamined Patent
Publication No. 2007-318077, it is possible to improve a manufacturing
yield.

[0016]In a method of manufacturing the semiconductor laser disclosed in
Japanese Unexamined Patent Publication No. 2007-318077, as shown in FIG.
17D, a heat treatment is performed while the ZnO layer 11 and the p-type
GaAs cap layer 9 are provided on the same layer so as to come into
contact with each other, thereby diffusing Zn in the ZnO layer 11 into
the active layer 4. In this case, Zn is diffused in the resonator
direction through the p-type GaAs cap layer 9. As a result, a non-gain
region of the active layer is widened, which causes an increase in
threshold current value.

[0017]When the volume of the non-gain region is large in the active layer,
a sufficient gain is not obtained, which results in an increase in
threshold current value. In particular, this phenomenon is remarkable in
the laser that has a relatively small resonator length and is for playing
back an optical disk. In addition, in devices using a fine balance
between the gain and the loss, such as self-oscillation lasers, it is
very difficult to generate self-oscillation.

[0018]As described above, the related art disclosed in Japanese Unexamined
Patent Publication Nos. 2006-319120 and 2007-318077 has the following
problems.

[0019]First, since impurities are diffused in the resonator direction of
the active layer, the non-gain region is widened, which causes an
increase in threshold current value.

[0020]Second, since a crystal defect occurs due to process damage, it is
difficult to control the alloying of the active layer with high
reproducibility.

SUMMARY

[0021]In one embodiment, there is provided a semiconductor laser
including: a substrate; and an active layer that is provided over the
substrate. The active layer in a region near at least one end surface is
alloyed by the diffusion of impurities. The band gap of the active layer
in the region near the end surface is wider than that of the active layer
in a region other than the region near the end surface. In this
semiconductor (λa-λw)>(more than) 15 (nm) and Lt is less
than 25 (μm), where"λw" (nm)" is the wavelength of light
corresponding to the band gap of the active layer disposed at a position
within a distance of 2 μm from one end surface in a resonator
direction, "λa (nm)" is the wavelength of light corresponding to
the band gap of the active layer disposed at a position that is spaced a
distance of equal to or more than ( 3/10)L and equal to or less than (
7/10)L from the one end surface in a resonator direction, "L" is the
resonator length, and "Lt" is the length of a transition region provided
between the position of the active layer with a band gap corresponding to
a light wavelength of λw+2 (nm) and the position of the active
layer with a band gap corresponding to a light wavelength of λa-2
(nm) in the resonator direction.

[0022]In a process of manufacturing the semiconductor laser in which
λa-λw>15 nm and Lt is less than 25 μm, it is possible
to reduce a threshold current value by reducing Lt and sharply changing
the band gap of the active layer in the region near the end surface.

[0023]In another embodiment, there is provided a method of manufacturing a
semiconductor laser including: forming an active layer over a substrate;
and diffusing impurities into a region near an end surface of the active
layer to alloy the active layer. The step of alloying the active layer
includes: preparing a semiconductor layer A and a semiconductor layer B;
sequentially forming the semiconductor layer B and the semiconductor
layer A over the active layer when the solid solubility limit
concentration of the semiconductor layer A with respect to the impurities
is Ma and the solid solubility limit concentration of the semiconductor
layer B with respect to the impurities is Mb (Ma>Mb); forming a groove
in the semiconductor layer A between a region where the region near the
end surface is to be formed and a region where a region other than the
region near the end surface is to be formed; and forming a layer
including the impurities so as to come into contact with only the surface
of the first semiconductor layer A in the region where the region near
the end surface is to be formed or exposing only the surface of the
semiconductor layer A in the region where the region near the end surface
is to be formed to a gas including the impurities, and diffusing the
impurities into the active layer in the region where the region near the
end surface is to be formed through the semiconductor layer A and the
semiconductor layer B.

[0024]According to the method of manufacturing a semiconductor laser, it
is possible to manufacture the semiconductor laser in which the band gap
of the active layer in the region near the end surface is sharply
changed.

[0025]That is, the groove is formed in the semiconductor layer A to
separate a region where the region near the end surface is to be formed
from a region where a region other than the region near the end surface
is to be formed, and impurities are diffused while a layer including
impurities or a gas including impurities comes into contact with only the
surface of the semiconductor layer A in the region where the region near
the end surface is to be formed. In this way, it is possible to prevent
impurities from being diffused into the semiconductor layer A in a
region, which will be the region other than the region near the end
surface, over the groove.

[0026]In addition, since Ma>Mb, impurity concentration is saturated in
a predetermined region or more of the semiconductor layer A, and
impurities are diffused into the semiconductor layer B. In other words,
it is possible to prevent impurities from being diffused into the
semiconductor layer B in the resonator direction at the same time as the
impurities start to be diffused into the semiconductor layer A.

[0027]In addition, the subsequent manufacturing process is performed with
the semiconductor layer A remaining in the region where the region other
than the region near the end surface is to be formed. Therefore, it is
possible to prevent the occurrence of a crystal defect due to process
damage.

[0028]In still another embodiment, there is provided a method of diffusing
impurities into a semiconductor layer B through a semiconductor layer A.
The method includes: preparing the semiconductor layer A and the
semiconductor layer B; forming the semiconductor layer A over the
semiconductor layer B when the solid solubility limit concentration of
the semiconductor layer A with respect to the impurities is Ma and the
solid solubility limit concentration of the semiconductor layer B with
respect to the impurities is Mb (Ma>Mb); forming a groove in the
semiconductor layer A, to form a first semiconductor layer A and a second
semiconductor layer A over the semiconductor layer B; and forming a layer
including the impurities so as to come into contact with only the surface
of the first semiconductor layer A and diffusing the impurities into the
semiconductor layer B through the first semiconductor layer A.

[0029]According to the above-mentioned embodiments of the invention, it is
possible to provide a structure capable of preventing the deterioration
of an end surface due to optical damage and producing a semiconductor
laser having a small threshold current with high yield and a method of
manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]The above and other objects, advantages and features of the present
invention will be more apparent from the following description of certain
preferred embodiments taken in conjunction with the accompanying
drawings, in which:

[0031]FIG. 1 is a perspective view illustrating the structure of a
semiconductor laser according to a first embodiment of the invention;

[0032]FIG. 2 is a perspective view illustrating the structure of the
semiconductor laser according to the first embodiment of the invention;

[0033]FIGS. 3A and 3B are diagrams illustrating a process of manufacturing
the semiconductor laser according to the first embodiment of the
invention;

[0034]FIGS. 4A and 4B are diagrams illustrating the process of
manufacturing the semiconductor laser according to the first embodiment
of the invention;

[0035]FIG. 5 is a graph illustrating a variation in the photoluminescence
wavelength (band gap) of an active layer in a resonator direction from
the end surface of the semiconductor laser;

[0036]FIGS. 6A and 6B are histograms illustrating the coherence
(self-oscillation intensity) of the semiconductor laser;

[0037]FIGS. 7A and 7B are diagrams illustrating the difference between a
Zn diffusion process of a method of manufacturing the semiconductor laser
according to the embodiment of the invention and a Zn diffusion process
of a method of manufacturing the semiconductor laser according to a
comparative example;

[0038]FIG. 8 is a perspective view illustrating the structure of a
semiconductor laser according to a second embodiment of the invention;

[0039]FIG. 9 is a perspective view illustrating the structure of the
semiconductor laser according to the second embodiment of the invention;

[0040]FIGS. 10A and 10B are diagrams illustrating a process of
manufacturing the semiconductor laser according to the second embodiment
of the invention;

[0041]FIG. 11 is a diagram illustrating the process of manufacturing the
semiconductor laser according to the second embodiment of the invention;

[0042]FIGS. 12A and 12B are diagrams illustrating a process of
manufacturing a semiconductor laser according to a third embodiment of
the invention;

[0043]FIGS. 13A and 13B are diagrams illustrating a process of
manufacturing the semiconductor laser according to the comparative
example;

[0044]FIGS. 14A and 14B are diagrams illustrating the process of
manufacturing the semiconductor laser according to the comparative
example;

[0045]FIGS. 15A and 15B are diagrams illustrating the process of
manufacturing the semiconductor laser according to the comparative
example;

[0046]FIGS. 16A to 16C are diagrams illustrating a process of
manufacturing a semiconductor laser according to the related art; and

[0047]FIGS. 17A to 17D are diagrams illustrating a process of
manufacturing a semiconductor laser according to the related art.

DETAILED DESCRIPTION

[0048]The invention will be now described herein with reference to
illustrative embodiments. Those skilled in the art will recognize that
many alternative embodiments can be accomplished using the teachings of
the present invention and that the invention is not limited to the
embodiments illustrated for explanatory purposes.

[0049]Hereinafter, exemplary embodiments of the invention will be
described with reference to the accompanying drawings. In the drawings,
the same components are denoted by the same reference numerals and
description thereof will not be repeated.

First Embodiment

[0050]A first embodiment of the invention will be described with reference
to FIGS. 1 and 2.

[0051]FIG. 1 is a perspective view illustrating the structure of a
semiconductor laser according to the first embodiment. FIG. 2 is a
perspective view illustrating the semiconductor laser when a portion of
the structure shown in FIG. 1 is removed to facilitate understanding of
an internal structure. In the drawings, a dark color portion indicates a
Zn-diffused region 511.

[0052]The semiconductor laser according to the first embodiment is a
self-oscillation semiconductor laser having a window structure at the end
thereof.

[0053]The semiconductor laser includes a substrate (n-type GaAs substrate
501) and an active layer (quantum well active layer 504) that is provided
on the n-type GaAs substrate 501. In the window structure of the
semiconductor laser, the quantum well active layer 504 of at least one
region near an end surface is alloyed by the diffusion of impurities, the
band gap of the quantum well active layer 504 of the region near the end
surface is wider than that of the quantum well active layer 504 of
regions other than the region near the end surface. In this embodiment,
Zn is used as the impurities, but the invention is not limited thereto.
Various kinds of metal materials may be used as the impurities.

[0054]As shown in FIG. 1, in the Zn-diffused region 511, Zn is diffused
and the concentration of Zn is increased. In the Zn-diffused region 511,
the band gap of the quantum well active layer 504 is wider than those of
the other portions.

[0055]In the semiconductor laser, the wavelength of light corresponding to
the band gap of the quantum well active layer 504 disposed at a position
within a distance of 2 μm from one end surface in the resonator
direction is λw (nm), a resonator length is L, and the wavelength
of light corresponding to the band gap of the quantum well active layer
504 disposed at a position that is spaced a distance of equal to or more
than ( 3/10)L and equal to or less than ( 7/10)L from the other end
surface in the resonator direction is λa (nm). In addition, a
transition region is arranged between the position of the quantum well
active layer 504 with a band gap corresponding to a light wavelength of
λw+2 (nm) and the position of the quantum well active layer 504
with a band gap corresponding to a light wavelength of λa-2 (nm) in
the resonator direction, and the length of the transition region is Lt.
In this case, λa-λw>15 (in other word, "λa-λw"
is more than 15) nm and Lt is less than 25 μm.

[0057]In addition, a double hetero structure including the quantum well
active layer 504, the n-type AlGaInP clad layer 503, and the p-type
AlGaInP clad layer 505 is formed. A current block layer may be a single
layer or a multi-layer structure. In this embodiment, a multi-layer
structure (n-type AlInP/GaAs current block layer 513) is used as the
current block layer.

[0058]As shown in FIGS. 1 and 2, the n-type GaInP hetero-barrier reduction
layer 502, the n-type AlGaInP clad layer 503, the quantum well active
layer 504, the p-type AlGaInP clad layer 505, the p-type GaInP
hetero-barrier reduction layer 506, the p-type GaAs cap layer 507, and
the p-type GaAs contact layer 514 are stacked on the n-type GaAs
substrate 501. In addition, a ridge portion (ridge-shape stripe) is
formed on the p-type AlGaInP clad layer 505. A current narrowing
structure including the n-type AlInP/GaAs current block layer 513 is
formed on side of a ridge (a side wall of the ridge-shaped p-type AlGaInP
clad layer 505). The n electrode (not shown) is formed below the n-type
GaAs substrate 501 and the p electrode is formed on the p-type GaAs
contact layer 514.

[0059]As shown in FIG. 2, a groove (separation groove) is formed in the
p-type GaAs cap layer 507 between the region near the end surface and a
region other than the region near the end surface. The groove causes the
p-type GaAs cap layer 507 in which Zn is diffused to be formed in the
region near the end surface and causes the p-type GaAs cap layer 507 that
does not include Zn, expect for that included in the processes before a
Zn diffusion process, to be formed on the quantum well active layer 504
of a gain region.

[0060]According to the above-mentioned structure, it is possible to form a
self-oscillation semiconductor laser in which a saturable absorption
region is formed on side of a ridge. In addition, it is possible to
appropriately adjust a difference in refractive index between a ridge
portion and both on side of the ridge and a ridge width. The resonator
length L is not particularly limited. For example, the resonator length L
may be equal to or less than 500 μm. The semiconductor laser according
to this embodiment may have a ridge-shaped stripe or a ridge-buried
stripe.

[0061]Next, a method of manufacturing the first embodiment will be
described with reference to FIGS. 3A and 3B and FIGS. 4A and 4B.

[0062]FIGS. 3A and 3B and FIGS. 4A and 4B are diagrams illustrating the
procedure of the method of manufacturing the semiconductor laser
according to the first embodiment. In the drawings, a portion
corresponding to one semiconductor laser element in the wafer is shown.
However, in the actual manufacturing process, a plurality of elements is
two-dimensionally arranged on the wafer.

[0063]The method of manufacturing the semiconductor laser according to
this embodiment includes a process of forming an active layer (quantum
well active layer 504) on a substrate (n-type GaAs substrate 501), a
process of diffusing impurities into a region near the end surface of the
quantum well active layer 504, to alloy the quantum well active layer
504. In this way, it is possible to form a window structure in the
quantum well active layer 504 of the region near the end surface in which
the impurities are diffused.

[0064]The process of alloying the quantum well active layer 504 includes
the following processes (1) to (3):

[0065]Process (1): a semiconductor layer A and a semiconductor layer B are
prepared, the solid solubility limit concentration of the semiconductor
layer A with respect to impurities is Ma, the solid solubility limit
concentration of the semiconductor layer B with respect to impurities is
Mb (Ma>Mb, that is Ma is lager than Mb), and the semiconductor layer B
and the semiconductor layer A are sequentially formed on the quantum well
active layer 504;

[0066]Process (2): a groove is formed in the semiconductor layer A between
a region where the region near the end surface is to be formed and a
region where the region other than the region near the end surface is to
be formed; and

[0067]Process (3): a layer including impurities is formed so as to come
into contact with only the surface of the semiconductor layer A in the
region where the region near the end surface is to be formed, and
impurities are diffused into the quantum well active layer 504 in the
region where the region near the end surface is to be formed through the
semiconductor layer A and the semiconductor layer B.

[0068]The semiconductor layer A may be made of any material as long as the
material includes GaAs. The semiconductor layer B may be made of any
material as long as the material includes GaInP, AlGaInP or AlGaAs.

[0069]In this embodiment, the semiconductor layer A is the p-type GaAs cap
layer 507, the semiconductor layer B is the p-type AlGaInP clad layer
505, and the impurities are Zn.

[0070]Next, the processes (1), (2) and (3) will be described in detail.

[Process (1)]

[0071]First, the n-type GaInP hetero-barrier reduction layer 502
(thickness: 0.02 μm), the n-type AlGaInP clad layer 503 (thickness:
1.2 μm), the quantum well active layer 504 (a septuple quantum well
including a GaInP well layer with a thickness of 0.006 μm and an
AlGaInP barrier layer with a thickness of 0.005 μm), the p-type

[0073]Then, a SiO2 film (thickness: 0.1 μm) is formed on the
p-type GaAs cap layer 507 by a chemical vapor deposition (CVD) method,
and a portion of the SiO2 film is opened by photolithography and wet
etching.

[0074]The length of the opening in the resonator direction is 7.5 μm,
and one end of the opening close to the end surface is spaced a distance
of 12.5 μm from the end surface. A portion of the p-type GaAs cap
layer 507 disposed immediately below the opening is removed by wet
etching.

[0075]In this way, as shown in FIG. 3A, a groove is formed in the p-type
GaAs cap layer 507 in a direction orthogonal to the resonator direction.
Therefore, it is possible to separate a region where the region near the
end surface is to be formed and a region where the region other than the
region near the end surface (gain region) is to be formed in the p-type
GaAs cap layer 507.

[Process (3)]

[0076]Then, after the SiO2 film is completely removed, a SiO2
film 508 (thickness: 0.2 μm) is formed by the CVD method, and an
opening is formed in the vicinity of the center of an end surface when
the film is cleaved to a laser chip. In this case, the length of the
opening in the resonator direction is 20 μm.

[0077]Then, a ZnO film 509 (thickness: 0.1 μm) and a SiO2 film 510
(thickness: 0.1 μm) are formed by a sputtering method.

[0078]In this way, as shown in FIG. 3A, it is possible to form a layer
(SiO2 film 508) for preventing the diffusion of impurities, in the
groove. The SiO2 film 508 may be formed so as to fill up the groove.
The SiO2 film 508 prevents Zn from being diffused into the p-type
GaAs cap layer 507 in the resonator direction. The layer for preventing
the diffusion of impurities is not limited to the SiO2 film 508, but
various kinds of material may be used.

[0079]Then, a heat treatment is performed at a temperature of 580°
C. for 20 minutes to diffuse Zn from the ZnO film 509 to the p-type GaAs
cap layer 507, the p-type GaInP hetero-barrier reduction layer 506, and
the quantum well active layer 504 and diffuse Zn to a depth of about 0.2
μm from the surface of the quantum well active layer 504 (FIG. 3A).

[0080]In this case, since the ZnO film 509 is contacted with the
semiconductor on side of the end surface than a region in which the
p-type GaAs cap layer 507 is removed, Zn is not diffused to the inside of
the resonator over the removed region (groove). Since the surface of the
p-type GaInP hetero-barrier reduction layer 506 is less stable with
respect to a process, such as a heat treatment or a plasma process, than
the surface of the p-type GaAs cap layer 507, the size of the region in
which the p-type GaAs cap layer 507 is removed is preferably small, other
than 0. When the region is applied to a semiconductor laser, the size of
the region in the resonator direction is preferably in the range 2 to 15
μm. In addition, it is preferable that a film coming into contact with
the semiconductor with a large area, such as the SiO2 film 508, be
formed by a CVD method that causes less damage to the film than a
sputtering method.

[0081]In the next process of this embodiment, the p-type GaAs cap layer
507 may remain on the p-type AlGaInP clad layer 505 in a region where the
region other than the region near the end surface is to be formed (FIG.
2). In addition, the p-type GaAs cap layer 507 may remain on the p-type
AlGaInP clad layer 505 in a region where the region near the end surface
is to be formed (FIG. 2). In this way, it is possible to reduce process
damage.

[0082]Then, the SiO2 film 508, the ZnO film 509, and the SiO2
film 510 are removed by wet etching, and a new SiO2 film 512
(thickness: 0.2 μm) is formed. Then, the SiO2 film 512 is
processed into a strip with a width of 3 μm to 5 μm by
photolithography and wet etching, and the semiconductor is removed up to
a position of 0.2 μm to 0.4 μm above the surface of the quantum
well active layer 504 by wet etching. In this way, a ridge-shaped strip
is formed (FIG. 3B).

[0083]Then, the n-type AlInP/GaAs current block layer 513 (thickness: 1.0
μm) made of n-type AlInP or n-type GaAs, or a combination thereof is
stacked on both sides of the ridge by an MOCVD method while the
strip-shaped SiO2 film 512 remains (FIG. 4A).

[0085]Then, the p electrode is formed on the p-type GaAs contact layer
514, and the entire wafer is processed such that the thickness thereof is
about 100 μm. Then, the n electrode is formed on the lower surface of
the n-type GaAs substrate 501. Then, the wafer is cleaved such that the
region having Zn diffused therein (Zn-diffused region 511) includes the
end surface, and is divided into individual semiconductors. The
semiconductor laser chip according to this embodiment is obtained by the
above-mentioned processed.

[0086]Next, the mechanism that the lateral diffusion of Zn is improved
when there is the p-type GaAs cap layer 507 is partially removed will be
described with reference to Table 1.

[0087]Table 1 shows the solid solubility limit concentration of Zn with
respect to GaAs and AlGaInP respectively and the Zn concentrations of the
p-type GaAs cap layer 507 and the p-type AlGaInP clad layer 505 provided
in this embodiment before diffusing Zn. As can be seen from Table 1, the
solid solubility limit concentration of Zn with respect to GaAs is lager
than the Zn concentration of the p-type GaAs cap layer 507, but the solid
solubility limit concentration of Zn with respect to AlGaInP is nearly
equal to the Zn concentration of the p-type AlGaInP clad layer 505. Thus,
the amount of Zn to be additionally doped into the p-type GaAs cap layer
507 is large, but the amount of Zn to be additionally doped into the
p-type AlGaInP clad layer 505 is small. Therefore, Zn diffused from the
ZnO film 509 to the p-type GaAs cap layer 507 is hardly diffused into the
p-type GaInP hetero-barrier reduction layer 506 or the p-type AlGaInP
clad layer 505 and is easily diffused into the p-type GaAs cap layer 507
in the lateral direction. When the concentration of Zn is saturated over
a wider region than intended of the p-type GaAs cap layer 507, Zn begins
to be diffused into the p-type GaInP hetero-barrier reduction layer 506
and the p-type AlGaInP clad layer 505 in a vertical direction so as to
push out Zn which presents in the p-type GaInP hetero-barrier reduction
layer 506 or the p-type AlGaInP clad layer 505 before diffusing Zn.

[0088]In the process of this invention, by blocking the path of Zn
diffusion along the resonator direction, that is achieved by partially
removing the p-type GaAs cap layer 507, it is possible to prevent Zn from
being sequentially diffused into the p-type AlGaInP clad layer 505 in the
resonator direction.

[0089]In Table 1, the solid solubility limit concentration of the p-type
GaAs cap layer is based on data disclosed in Journal of Crystal Growth,
167 (1996), p. 17. The document discloses a method of estimating the
solid solubility limit concentration of Zn with respect to GaAs from the
concentration equilibrium between a vapor phase including a Zn compound
and a solid phase composed of a GaAs semiconductor including Zn as
impurities.

[0090]Moreover, in Table 1, the solid solubility limit concentration of
the p-type AlGaInP layer is obtained by the experiments by the inventors.
The solid solubility limit concentration of Zn to AlGaInP was estimated
from a Zn concentration profile in the stacked direction by SIMS
measurement. The method used in the estimation is disclosed in Physical
Status Solid (a), 149 (1995), p. 557.

[0091]Further, the Zn concentrations of the p-type GaAs cap layer 507 and
the p-type AlGaInP clad layer 505 provided in this embodiment before
diffusing Zn are measured by SIMS measurement.

[0092]Next, the measurement result of the wavelength of light
corresponding to the band gap of the active layer will be described.

[0093]FIG. 5 is a graph illustrating the measurement results of the
wavelengths of light corresponding to the band gaps of the active layers
of the semiconductor laser ((a) of FIG. 5) according to this embodiment
and a semiconductor laser ((b) of FIG. 5) according to a comparative
example, which will be described below. In the graph, the horizontal axis
indicates the distance from the end surface in the resonator direction,
and the vertical axis indicates the peak wavelength of photoluminescence
light. In addition, it is assumed that the wavelength of light at an end
surface position (a position where the horizontal axis is 0 μm) is
λw (nm), and the wavelength of light at the position where the
horizontal axis is 100 μm is λa (nm). In FIG. 5, Lt indicates
the length of the transition region.

[0094]The wavelength λw and the wavelength λa are the
wavelength of light corresponding to the band gap of the quantum well
active layer 504 at any position in each range. The wavelength of light
is obtained by measuring the peak wavelength of photoluminescence light.

[0095]In this embodiment, the wavelength λa may be measured in a
gain region at a position that is spaced a distance of equal to or more
than ( 3/10)L and equal to or less than ( 7/10)L, or a distance of equal
to or more than ( 4/10)L and equal to or less than ( 6/10)L from one end
surface in the resonator direction. For example, the wavelength λa
may be measured at any position in the distance range of equal to or more
than 100 μm and equal to or less than 150 μm from the end surface
in the resonator direction.

[0096]For example, a light absorbing layer, which is an obstacle to
measurement, is partially removed for measurement, if necessary.

[0097]Since the band gap of the active layer in the transition region is
different from that in the gain region, a gain for laser oscillation is
not generated in the transition region. In addition, since the absorption
spectrum of the semiconductor is spread from the peak wavelength,
absorption loss with respect to laser oscillation light generated from
the gain region, occurs in a portion of the transition region. Therefore,
in order to effectively perform laser oscillation, it is preferable that
Lt be small.

[0098]As shown in (a) of FIG. 5, in the semiconductor laser according to
this embodiment, the wavelength varies greatly as the distance along the
horizontal axis increases. In contrast, as shown in (b) of FIG. 5, in the
semiconductor laser according to comparative example, the wavelength
varies moderately as the distance along the horizontal axis increases.

[0099]The length of the transition region, Lt is 12 μm in (a) of FIG. 5
and 46 μm in (b) of FIG. 5.

[0100]As such, in the semiconductor laser according to this embodiment,
the length of the transition region, Lt is smaller than that in the
comparative example. Therefore, it is possible to prevent absorption loss
and reduce a threshold current value.

[0101]Next, the operation and effects of this embodiment will be
described.

[0102]The semiconductor laser according to this embodiment has a window
structure in which the quantum well active layer 504 in the regions near
both end surfaces is alloyed and the band gap of the quantum well active
layer 504 in the region near the end surface is wider than that of the
quantum well active layer 504 in the gain region (region other than the
region near the end surface). Since the region with a wider band gap
functions as a transparent window that does not absorb laser oscillation
light, it is possible to significantly increase the level where optical
damage (COD) occurs. In this way, it is possible to achieve a
semiconductor laser with high stability.

[0103]In the semiconductor laser, if λa-λw>15 nm, it is
possible to prevent the occurrence of COD.

[0104]Since λa-λw>15 nm and the length of the transition
region, Lt is less than 25 μm, as shown in FIG. 5, the band gap of the
quantum well active layer 504 varies sharply in the resonator direction
from both ends. Therefore, it is possible to narrow a non-gain region in
which the gain is not generated and widen the gain region. In addition,
it is possible to prevent propagation loss. As a result, it is possible
to obtain a sufficient gain and reduce a threshold current value. In
particular, these effects are remarkable in a laser that has a relative
short resonator length and is for playing back an optical disk.

[0105]As the length of the transition region, Lt is smaller, loss due to
light absorption is reduced. However, when the Lt is excessively small, a
large variation in refractive index occurs at the boundary between the
diffusion and non-diffusion regions, and the loss of guided light is
increased due to scattering. Therefore, Lt is preferably equal to or more
than 1 μm, and more preferably, equal to or more than 3 μm.

[0106]In this embodiment, in the case of a self-oscillation semiconductor
laser, it is preferable that the length of the transition region, Lt be
equal to or less than 12 μm and equal to or more than 1 μm.

[0107]As described above, in the semiconductor laser of this embodiment in
which the band gap of the active layer near the end surface is wide
enough not to absorb laser oscillation light, it is possible to achieve a
structure capable of reducing a threshold current value and obtaining
stable self-oscillation by narrowing the region in which the band gap of
the active layer varies in the resonator direction from the end surface,
that is, sharply changing the band gap of the region near the end
surface.

[0108]If Lt is less than 25 μm but the value of ha-λw is equal to
or less than 15 nm, the difference between the band gap of the non-gain
region and the band gap of the gain region is reduced, and it is
difficult to sufficiently prevent the occurrence of COD.

Comparative Example

[0109]Next, a semiconductor laser according to the comparative example
which corresponds to the semiconductor laser disclosed in Japanese
Unexamined Patent Publication No. 2007-318077 will be described. The
semiconductor laser according to the comparative example is manufactured
by the following process.

[0110]First, as shown in FIG. 13A, an n-type GaInP hetero-barrier
reduction layer 1002, an n-type AlGaInP clad layer 1003, a quantum well
active layer 1004, a p-type AlGaInP clad layer 1005, a p-type GaInP
hetero-barrier reduction layer 1006, and a p-type GaAs cap layer 1007 are
sequentially stacked on an n-type GaAs substrate 1001 by one epitaxial
growth, and a SiO2 film 1008 is formed on the p-type GaAs cap layer
1007 by CVD and photolithography. The SiO2 film 1008 is opened in a
region corresponding to near the end surface of an LD chip. Then, a ZnO
film 1009 and a SiO2 film 1010 are formed by a sputtering method.
Then, when the product is heated at a temperature of about 600° C.
for 10 to 30 minutes, Zn atoms are diffused from a contact portion
between the p-type GaAs cap layer 1007 and the ZnO film 1009 to a
semiconductor, and a Zn diffused region 1011 represented by a deep-color
portion shown in FIG. 13A is formed. Then, the SiO2 film 1008, the
ZnO film 1009, and the SiO2 film 1010 are removed. Then, as shown in
FIG. 13B, the semiconductor is removed to a depth of 0.2 to 0.4 μm
from the upper surface of the quantum well active layer 1004 by wet
etching or dry etching using a SiO2 strip 1012 that is formed by CVD
and photolithography as an etching mask, thereby forming a ridge. Then, a
resist film 1015 is formed as shown in FIGS. 14A and 14B, and portions of
the SiO2 strip 1012 and the p-type GaAs cap layer 1007 near the end
surface are removed by photolithography and etching. Then, the resist
film 1015 is removed and an n-type current block layer 1013 and a p-type
contact layer 1014 are formed by second and third epitaxial growth
processes, as shown in FIGS. 15A and 15B. Then, the upper and lower
electrodes are formed, and the semiconductor is divided such that the
Zn-diffused region 1011 includes the end surface. In this way, a
semiconductor laser chip according to the comparative example is
obtained.

Comparison Between Embodiment and Comparative Example

[0111]Next, the effects of this embodiment compared to the comparative
example will be described.

[0112]When the semiconductor laser according to the comparative example is
manufactured by the above-mentioned manufacturing process, Zn is diffused
in the resonator direction, the non-gain region of the active layer is
widened, and the length of the transition region, Lt is equal to or more
than 25 μm. This causes an increase in threshold current value.

[0113]That is, in the semiconductor laser according to the comparative
example, if Lt is equal to or more than 25 μm and the band gap of the
active layer is slowly reduced from the end surface to the inside of the
resonator, the area of the non-gain region is increased, and the gain is
likely to be insufficient, which results in an increase in threshold
current value. In particular, in the laser for playing back an optical
disk that has a relatively small resonator length, this phenomenon is
remarkable. In addition, in a device using a fine balance between gain
and loss, such as the self-oscillation laser, it is likely difficult for
self-oscillation to occur.

[0114]In contrast, in the process of manufacturing the semiconductor laser
according to this embodiment, as described above, since
λa-λw>15 nm and Lt is less than 25 μm, it is possible
to achieve a small semiconductor laser. Therefore, the band gap of the
quantum well active layer 504 varies greatly in the resonator direction
from the end surface. In this way, it is possible to narrow the non-gain
region and widen the gain region. In addition, it is possible to prevent
propagation loss. As a result, it is possible to obtain a sufficient gain
and reduce a threshold current value. In particular, the effects are
remarkable in the laser for playing back an optical disk that has a
relative small resonator length. Further, in this embodiment, it is
possible to maintain the balance between the gain and the loss.
Therefore, it is possible to achieve stable self-oscillation in the
self-oscillation semiconductor laser.

[0115]Next, the effects of the manufacturing method according to this
embodiment compared to that of the manufacturing method according to the
comparative example will be described.

[0116]The process of manufacturing the semiconductor laser according to
this embodiment is different from the process of manufacturing the
semiconductor laser according to the comparative example in a Zn
diffusing process (FIGS. 7A and 7B). The curves of the photoluminescence
peak wavelengths shown in (a) and (b) of FIG. 5 correspond to the
structures shown in FIGS. 7A and 7B, respectively.

[0117]In the process of manufacturing the semiconductor laser according to
the comparative example, as shown in FIG. 7B, the p-type GaAs cap layer
1007 is formed above the entire surface of the quantum well active layer
1004. Therefore, when a heating process is performed with the p-type GaAs
cap layer 1007 and the ZnO film 1009 coming into contact with each other,
Zn is diffused into the quantum well active layer 1004 in the resonator
direction through the p-type GaAs cap layer 1007. Then, the Zn diffused
region 1011 is widened, and the non-gain region of the quantum well
active layer 1004 is also widened.

[0118]In contrast, in the process of manufacturing the semiconductor laser
according to this embodiment, as shown in FIG. 7A, a portion of the
uppermost p-type GaAs cap layer 507 is removed and a separation groove is
formed. The separation groove may separate a region where the region near
the end surface is to be formed from a region where the gain region is to
be formed in the p-type GaAs cap layer 507. Since the ZnO film 509 comes
into contact with only the p-type GaAs cap layer 507 in the region where
the region near the end surface is to be formed, it is possible to
prevent Zn from being diffused into the p-type GaAs cap layer 507 in the
region where the gain region is to be formed over the separation groove.
In addition, the SiO2 film 508 is inserted in the groove such that
Zn is not diffused into the p-type GaAs cap layer 507 in the region where
the gain region is to be formed.

[0119]As shown in FIGS. 7A and 7B, after the heat treatment, the portions
having Zn diffused thereinto become the Zn-diffused region 511 and the
Zn-diffused region 1011. In FIG. 7B, since the p-type GaAs cap layer is
not cut, a large amount of Zn is diffused in the lateral direction. In
contrast, in FIG. 7A, Zn is diffused to only the boundary from the
portion in which the p-type GaAs cap layer is removed in the lateral
direction. Therefore, the length of the Zn-diffused region 511 according
to this embodiment of the invention in the resonator direction is smaller
than that of the Zn-diffused region 1011 of the comparative example.

[0120]As such, the diffusion of Zn of this embodiment in the resonator
direction is suppressed than that of the comparative example. Therefore,
in the manufacturing process according to this embodiment, it is possible
to reduce the length of the transition region, Lt. That is, it is
possible to sharply change the band gap of the active layer from the
window region to the inside of the resonator.

[0121]In this embodiment, even when the p-type GaAs cap layer 507 is
entirely removed to form the ZnO film 509 and the SiO2 film 508, the
diffusion of Zn in the lateral direction is prevented. However, when a
layer (for example, the p-type AlGaInP clad layer 505 or the p-type GaInP
hetero-barrier reduction layer 506) made of a semiconductor, such as a
GaInP layer or a AlGaInP layer, other than the GaAs layer (p-type GaAs
cap layer 507) is exposed, the structure is likely to be damaged by, for
example, a heat treatment and a plasma process in the manufacturing
process, and a defect may spread from the surface of the semiconductor to
the inside, which may result in the alloying of the quantum well active
layer 504 and a widening in the band gap. In this embodiment, the ZnO
film 509, which is a Zn diffusion source, is easily formed by the
sputtering method. However, a semiconductor crystal as a base, is more
damaged by the sputtering method than by the CVD method. Therefore, the
sputtering method requires special care. In consideration of the above
points, the p-type GaAs cap layer 507 may not be partially removed in
this embodiment.

[0122]As described above, in this embodiment, after the p-type GaAs cap
layer 507 is partially removed, Zn is diffused. Therefore, the diffusion
of Zn in the resonator direction is prevented. In addition, the region in
which the p-type GaAs cap layer 507 is removed is minimized. Therefore,
it is possible to prevent the deterioration of crystal quality, such as a
semiconductor defect during a heat treatment and a plasma process, and
prevent the band gap of the active layer in an unnecessary region from
being widened.

[0123]Next, the effects of this embodiment compared to the comparative
example in an AlGaInP-based semiconductor laser using a self-oscillation
operation will be described with reference to FIGS. 6A and 6B.

[0124]FIGS. 6A and 6B are graphs illustrating the results when an
interference spectrum, which is an index for the stability of
self-oscillation, is calculated and coherence, which is the ratio between
the primary (zero-order) peak intensity and the first-order peak
intensity of the interference spectrum, is calculated. In the graphs, the
measured values of 100 or more semiconductor laser elements are
represented as histograms. In general, the smaller the coherence becomes,
the more stable a self-oscillation operation becomes.

[0125]FIG. 6A shows the results obtained from the structure having a small
Lt according to this embodiment, and FIG. 6B shows the results obtained
from the structure having a large Lt according to the comparative
example. The graphs correspond to elements having the same structure as
the elements having the photoluminescence peak wavelengths shown in (a)
and (b) of FIG. 5.

[0126]In the structure of the semiconductor laser according to the
comparative example, since the coherence is large, the self-oscillation
operation is unstable.

[0127]In contrast, in the structure of the semiconductor laser according
to this embodiment, since the coherence is small, the self-oscillation
operation is stable.

[0128]As described above, in the semiconductor laser according to this
embodiment, it is possible to achieve stable self-oscillation by reducing
Lt and finely balancing the gain and the loss.

[0129]It is preferable that the length of the transition region, Lt be
small in terms of the following points: when the loss region or the
non-gain region is widened, it is likely difficult to obtain
self-oscillation; self-oscillation is likely to be affected by the
magnitude relationship between the gain and the loss; and other
characteristic points.

Second Embodiment

[0130]Next, a second embodiment of the invention will be described with
reference to FIGS. 8 to 11.

[0131]FIG. 8 is a perspective view illustrating the structure of a
semiconductor laser according to the second embodiment. FIG. 9 is a
perspective view illustrating the semiconductor laser when a portion of
the structure shown in FIG. 8 is removed to facilitate understanding of
the internal structure thereof.

[0132]FIGS. 10A and 10B and FIG. 11 are diagrams illustrating a process of
manufacturing the semiconductor laser according to the second embodiment.

[0133]The second embodiment differs from the first embodiment in that a
current block structure is provided over an upper part of the p-type
AlGaInP clad layer 505 in the region near the end surface.

[0134]In the second embodiment, after the process corresponding to FIG. 3B
in the first embodiment is conducted, a process shown in FIG. 10A is
performed. Specifically, the SiO2 film 512 and the p-type GaAs cap
layer 507 disposed from the concave portion of the p-type GaAs cap layer
507 shown in FIG. 3B to the end surface are removed by, for example, a
photolithography method. Then, an n-type current block layer 515 is
formed on the p-type AlGaInP clad layer 505 (FIG. 10B).

[0135]In this way, it is possible to remove the p-type GaAs cap layer 507
on the p-type AlGaInP clad layer 505 and form the n-type current block
layer 515 on the p-type AlGaInP clad layer 505 in the region near the end
surface.

[0136]After the SiO2 film 512 is removed, a p-type GaAs contact layer
516 is formed (FIG. 11). Then, a laser chip is manufactured by the same
process as that in the first embodiment. In this way, it is possible to
obtain the semiconductor laser according to the second embodiment.

[0137]In the first embodiment, in the region near the end surface, since
Zn is diffused downward, a pn junction portion is in the middle of the
n-type AlGaInP clad layer 503. In a general structure using the
semiconductor laser, the band gap of the clad layer is wider than that of
the active layer. The second embodiment also has this structure.
Therefore, even when a current block structure is not provided near the
end surface, a current is less likely to flow through the region near the
end surface than other regions, because the end surface portion has a
so-called remote junction. However, when a voltage with a predetermined
level or more is applied, a pn junction is turned on near the end surface
having a remote junction and a current may flow. This phenomenon easily
occurs in the voltage range that is generally used by the semiconductor
laser, but the gain is not generated near the end surface where the band
gap of the active layer is large even when a current is injected. As a
result, the threshold current value of the semiconductor laser increases.
In addition, the flow of a current near the end surface accelerates the
deterioration of the end surface, which may prevent the long-time
operation of the semiconductor laser. In order to further improve the
effects obtained by the window structure, it is preferable that a current
block structure be provided near the end surface, as in the second
embodiment.

[0138]As other embodiments, a so-called ridge structure may be used in
which the n-type AlInP/GaAs current block layer 513 and the p-type GaAs
contact layer 514, or the n-type current block layer 515 and the p-type
GaAs contact layer 516 are stacked.

Third Embodiment

[0139]Next, a third embodiment of the invention will be described with
reference to FIGS. 12A and 12B.

[0140]FIGS. 12A and 12B are diagrams illustrating a process of
manufacturing a semiconductor laser according to the third embodiment of
the invention.

[0141]In the third embodiment, the same process as that in the first
embodiment shown in FIG. 3B is conducted. After the process, as shown in
FIG. 12A, a region where the region near the end surface is to be formed
in the SiO2 film 512 and the p-type GaAs cap layer 507 is removed by
CVD, photolithography, and etching. Then, as shown in FIG. 12B, a
photoresist 517 and the SiO2 film 512 are entirely removed, and the
SiO2 film 518 only in a region on which current injection is desired
to be performed is opened by CVD, photolithography, and etching. Then, a
laser chip is obtained by the same process as that in the first
embodiment. In this way, it is possible to obtain the semiconductor laser
according to the third embodiment.

[0142]In the first and second embodiments, a thermal history corresponding
to crystal growth after forming the ridge is added. However, in the third
embodiment, since there is no addition of the thermal history, it is
possible to easily control Zn diffusion.

[0143]However, it is preferable to use a low-damage process such as CVD
for the formation of a SiO2 film on both sides of the ridge after
the p-type AlGaInP clad layer 505 is exposed, in consider of preventing a
crystal quality from being reduced.

[0144]A method of diffusing impurities in the method of manufacturing the
semiconductor laser may be applied to a general case. That is, when there
are solid A and solid B having different solid solubility limits with
respect to an impurity C, the solid solubility limit of the solid A is
large, and the impurity C is diffused from the solid A to the solid B, it
is possible to prevent the impurity C from being diffused in the lateral
direction by removing a portion of the solid A.

[0145]That is, as the impurity diffusion method, a method of diffusing
impurities into a semiconductor layer B through a semiconductor layer A
may be used.

[0146]The impurity diffusion method includes the following processes (1)
to (3):

[0147]Process (1): the semiconductor layer A and the semiconductor layer B
are prepared, and when the solid solubility limit concentration of the
semiconductor layer A with respect to impurities is Ma and the solid
solubility limit concentration of the semiconductor layer B with respect
to the impurities is Mb (Ma>Mb), the semiconductor layer A is formed
on the semiconductor layer B;

[0148]Process (2): a groove is formed in the semiconductor layer A, and a
first semiconductor layer A and a second semiconductor layer A are formed
on the semiconductor layer B; and

[0149]Process (3): a layer including impurities is formed so as to come
into contact with only the surface of the first semiconductor layer A,
and the impurities are diffused into the semiconductor layer B through
the first semiconductor layer A.

[0150]According to the semiconductor laser of this embodiment, it is
possible to achieve a high COD level and a high electrostatic discharge
(ESD) level without damaging an oscillation threshold current,
efficiency, or self-excited operation characteristics.

[0151]The semiconductor laser according to the embodiment of the invention
may be used as, for example, a light source of an optical disk device. In
addition, the self-oscillation semiconductor laser according to the
embodiment of the invention may be used as a DVD playback light source.

[0152]In Process (3), an opening may be formed in the SiO2 film 508,
the opening may be exposed to gas including impurities, and the gas may
come into contact with the only surface of the p-type GaAs cap layer 507
in a region where the region near the end surface is to be formed. In
this way, it is possible to prevent the diffusion of Zn in the resonator
direction of the quantum well active layer 504, similar to the method
forming the ZnO film 509.

[0153]Any gas including Zn may be used as the gas including impurities.
For example, diethyl zinc (DEZ) may be used.

[0154]The above-described embodiments and a plurality of modifications may
be combined with each other without departing from the scope of the
invention. In the above-described embodiments and modifications, the
structure of each component has been described in detail, but the
structure may be changed in various ways without departing from the scope
of the invention.

[0155]It is apparent that the present invention is not limited to the
above embodiment, and may be modified and changed without departing from
the scope and spirit of the invention.